The materials at the heart of the study are moiré heterostructures formed from atomically thin layers of molybdenum diselenide and tungsten diselenide, commonly written as MoSe2 and WSe2. When these two ultra-thin semiconductors are stacked with a small rotational twist, the mismatch between their crystal lattices generates a long-wavelength repeating pattern known as a moiré superlattice. This periodic potential modulates the electronic structure of the combined system and gives rise to unusual light-emitting behavior, including excitons bound to the moiré potential and emission features that vary dramatically from one point on the sample to another. Such heterostructures have attracted intense interest for applications ranging from light-emitting devices to quantum technologies, precisely because their optical properties can be engineered through twist angle and layer composition.

Yet the same structural richness that makes moiré materials exciting also makes them difficult to interpret. Photoluminescence spectra measured across the surface of a MoSe2/WSe2 heterostructure typically display a dense thicket of overlapping peaks, each potentially influenced by multiple physical mechanisms at once. Conventional analysis proceeds by decomposing such spectra into individual peaks and assigning each one to a specific excitonic state or defect population. In moiré systems, however, the peaks overlap so heavily and respond to so many simultaneous influences that peak-by-peak assignment becomes uncertain at best and misleading at worst. The result is a persistent gap between the optical data researchers can collect and the material information they actually need to improve device quality and reproducibility.

Wakabayashi’s approach turns this problem on its head. Instead of trying to resolve the spectrum into its constituent peaks, the new framework tracks simple spectral descriptors, such as the peak energy and the average energy of the emission, and examines how these quantities change spatially across the sample. The central insight is that the correlations between these descriptors, mapped from point to point, encode information about the disorder landscape that produced them. Different spectral features respond in characteristically different ways to different kinds of imperfection, so the statistical relationships among descriptors act as a fingerprint of the underlying structural disorder, even when no individual peak can be cleanly assigned.

Applying this theoretical framework to descriptor correlations reported for a twisted MoSe2/WSe2 heterostructure, Wakabayashi found evidence for a hierarchy of disorder operating on two distinct levels. The first level corresponds to a smooth background variation that unfolds over distances of a few micrometers, a gentle undulation in the potential experienced by excitons as they move across the moiré superlattice. This large-scale component likely reflects slow spatial drifts in strain, twist angle, or electrostatic environment across the heterostructure. The second level is far finer and much more localized, arising from small defects or exciton-trapping sites scattered through the bilayer. These trap-like sites capture excitons at specific points and imprint sharp local signatures on the emission, superimposed on the smooth background.

The power of the method lies in the fact that these two disorder layers can be separated mathematically by comparing how the spectral features vary in space, without ever decomposing the spectra into individual peaks. Through a detailed theoretical analysis of the descriptor correlations, the underlying landscape of disorder in the bilayer material can be inferred in a way that is systematic and reproducible. In effect, the complex spectrum stops being an obstacle and becomes a resource: the very overlap and complexity that defeat peak-by-peak analysis contain, in their spatial statistics, a faithful record of the imperfections that generated them.

The significance of this capability extends well beyond a single material system. Tiny structural imperfections and hidden disorder can strongly affect how materials emit and interact with light, degrading the performance of light-emitting devices, broadening emission lines, and introducing variability that undermines reproducibility. In quantum technologies, where the precise optical behavior of individual excitons or localized emitters matters, uncontrolled disorder can be the difference between a functioning device and a failed one. A diagnostic that reads disorder directly from ordinary optical measurements offers a practical route to quality control that does not require destructive interrogation of the sample or speculative spectral assignments.

According to Wakabayashi, the framework points toward exactly that kind of practical diagnostic. This work could help researchers make better and more reproducible materials for light-emitting devices, optical sensors, and quantum technologies, he remarks. By turning photoluminescence mapping into a quantitative probe of hierarchical disorder, the method gives materials growers and device engineers a feedback tool: they can measure a sample optically, extract its disorder landscape, and adjust growth or fabrication conditions accordingly, all without relying on uncertain peak-by-peak spectral interpretation that has long hampered the field.

The study, titled Hierarchical disorder in moiré exciton photoluminescence probed by spectral-descriptor correlations, was published in Physical Review Research on 7 August 2026 and is featured as Research Highlights Vol. 97 by MANA, the center’s mechanism for independently showcasing exceptional research achievements. MANA, established in 2007 under the World Premier International Research Center Initiative of Japan’s Ministry of Education, Culture, Sports, Science and Technology, pioneers the concept of nanoarchitectonics, the design of innovative materials at the nanoscale, and maintains a strongly international and interdisciplinary research program aimed at advancing materials science for a sustainable future.

Looking ahead, the descriptor-correlation framework is expected to extend naturally to other two-dimensional semiconductors and related light-emitting materials, wherever complex spectra and hidden disorder conspire to obscure the physics. As moiré platforms mature from laboratory curiosities into candidates for real photonic and quantum devices, the ability to map their imperfections quickly, non-destructively, and without ambiguous spectral assignments may prove as important as the ability to create them. What was once spectral noise, the tangled forest of overlapping peaks, has become a readable map of the material’s inner terrain, and that shift in perspective may shape how the next generation of light-emitting materials is diagnosed, refined, and ultimately built.

Subject of Research: Optical diagnostics of hierarchical disorder in moiré exciton photoluminescence of two-dimensional semiconductor heterostructures

Article Title: Reading beyond the peaks: MANA study finds clarity in spectral complexity

Article References: Reading beyond the peaks: MANA study finds clarity in spectral complexity. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: moiré heterostructures, photoluminescence, two-dimensional semiconductors, MoSe2/WSe2, spectral descriptors, material disorder, excitons, NIMS, MANA, Physical Review Research, optical diagnostics, quantum technologies

The man behind the project, Prof. Dr Matthias Wollenhaupt, who leads the Ultrafast Coherent Dynamics research group at Oldenburg, describes the achievement as a genuine extension of what experimentalists can do. “With our method, we can generate electronic quantum states that have previously only been described theoretically and also make them spatially visible,” he explains. “We have thus expanded the experimental optics toolkit to include a new class of three-dimensional light fields.” That statement captures the essence of the result: for decades, theorists have predicted that fully three-dimensional light fields should enable new forms of quantum control, but creating such fields in the laboratory and using them to steer electrons has remained out of reach until now.

The technical trick at the heart of the experiment lies in the combination of two specially shaped femtosecond laser pulses. Femtosecond pulses are bursts of light lasting only a few millionths of a billionth of a second, so short that within the duration of a single pulse, chemical bonds barely have time to begin vibrating. The researchers used an interferometer to split their laser light into two beams of different colours, shaped each pulse with great precision, and then directed the two beams so that they intersected inside a vacuum chamber at a single point. Where the beams overlapped, their electromagnetic fields added together, producing a three-dimensional interference pattern whose geometry the team could control by adjusting the colours, phases, polarizations and relative timing of the two pulses.

“The fields oscillate in all three spatial directions, opening up new possibilities for investigating and controlling specific light-matter interactions,” explains Darius Köhnke, one of the two lead authors of the study and a PhD student in Wollenhaupt’s group. Conventional laser experiments typically rely on light that oscillates in one or two dimensions, which limits the kinds of quantum states that can be reached. A truly three-dimensional field, by contrast, can drive an electron in ways that no one-dimensional or two-dimensional field can, coupling the electron’s motion along all three axes simultaneously. This is precisely what allows the method to access quantum states that are forbidden or invisible to conventional excitation schemes.

To demonstrate the power of the technique, the team applied their three-dimensional light fields to potassium atoms, a workhorse of atomic physics experiments. They used the fields to selectively excite electrons within the atoms into higher-energy states, known as excited states, and then to release those electrons from the atoms entirely, a process called ionization. The properties of the liberated electrons carry a detailed fingerprint of the quantum states from which they were ejected, allowing the researchers to reconstruct what happened inside the atom. Crucially, by varying the parameters of the light field, they could choose which excited states to populate and which pathways the electrons would follow on their way out.

The experiment also functioned like an ultra-high-speed camera for quantum processes. Because the laser pulses are so short, the researchers were able to observe the changes in the electron states at closely spaced time intervals, capturing successive snapshots of the evolving quantum system. The principle resembles stroboscopic flash photography, in which a rapidly flashing lamp freezes fast motion into a sequence of still images. Stitched together, these snapshots form a movie of the electron states as they evolve under the influence of the three-dimensional field. This time-resolved view is what allowed the team to confirm that the theoretically predicted states were indeed being created and to make their structure spatially visible for the first time.

Beyond its fundamental appeal, the method is particularly promising for one of the most consequential problems in modern chemistry: the detection and control of chiral molecules. Chiral molecules come in two forms that are mirror images of each other but cannot be superimposed, much like a person’s left and right hands. This handedness is not a minor detail. Many biomolecules, including amino acids, carbohydrates and the active ingredients of medicinal products, are chiral, and the two mirror-image forms, called enantiomers, frequently have strikingly different biological effects. The body may metabolize one form beneficially while the other is inert or even harmful.

The dangers of ignoring molecular handedness are written into pharmaceutical history. The active ingredient thalidomide, marketed in the late 1950s and early 1960s under the brand name Contergan, exists in two enantiomeric forms: one causes severe birth defects when taken during pregnancy, while the other is comparatively harmless. Because the two forms are chemically almost identical, separating or distinguishing between them can be extremely difficult, and in the case of thalidomide the drug was sold as a mixture of both. A technique that could reliably sense and even selectively manipulate one handedness over the other would therefore be a powerful tool for drug development, quality control and fundamental biology alike.

Three-dimensional light fields may offer exactly that capability. “Theoretical studies show that three-dimensional light fields can also possess chiral properties,” Wollenhaupt explains. In other words, the light itself can be handed, twisting through space in a way that distinguishes left from right. Such chiral light fields could interact differently with the two enantiomers of a chiral molecule, providing a far more sensitive and selective probe than conventional methods, which typically rely on weak magnetic-dipole effects. The opportunities that three-dimensional light fields open up for investigating and controlling molecular chirality were recently highlighted by physicist Prof. Dr Olga Smirnova of the Max Born Institute for Nonlinear Optics in Berlin in an article published in the journal Science entitled “A New Age of Molecular Chirality”. The Oldenburg team writes that their current study lays an important foundation for such applications, bridging the gap between theoretical proposals and laboratory reality.

The work, reported under the title “Multiphoton ionization with three-dimensional light fields,” signals a broader shift in how physicists think about controlling matter with light. Rather than treating a laser beam as a simple one-dimensional oscillating field, the Oldenburg approach treats the full spatial structure of light as a designable parameter, something to be engineered pulse by pulse in order to write specific quantum states onto atoms and molecules. The same interferometric superposition scheme that produced the new states in potassium atoms can, in principle, be adapted to more complex systems, from chiral molecules in solution to surfaces and nanostructures. If the promise holds, the coming years could see three-dimensional light fields become a standard instrument in the quest to read out, and perhaps even exploit, the handedness of nature at the molecular scale, turning a long-standing theoretical prediction into a practical technology for chemistry, biology and medicine.

Subject of Research: Generation of three-dimensional light fields from superimposed femtosecond laser pulses to control electron quantum states and probe chiral molecules

Article Title: Using three-dimensional light fields to control electrons

Article References: Using three-dimensional light fields to control electrons. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: three-dimensional light fields, femtosecond laser pulses, quantum states, multiphoton ionization, chiral molecules, light-matter interactions, potassium atoms, quantum control, ultrafast optics, chiral sensing, Physical Review Research, University of Oldenburg